Poor bone health is a consequence of Spinal Muscular Atrophy (SMA). Bone mineral density is significantly reduced1,2 in up to 50% of patients with SMA.3 Immobility and muscle weakness in SMA are significant risk factors for low bone mineralization, but bone density is not directly correlated with severity of patients’ weakness.3 Animal models suggest that the loss of SMN protein underlying SMA may also directly impair the bone remodeling process.4 Dietary deficiencies can also create osteopenia; vitamin D and calcium intake were below recommended levels in over half of patients with SMA in one published investigation.5
The combination of low mobility, osteoporosis, and low vitamin D levels increases the risk of fractures in SMA.6 A consortium of SMA experts recommends prevention of fractures through a comprehensive bone health program including optimized nutrition (sufficient calcium and vitamin D), physical therapy to avoid disuse, monitoring of bone density with imaging, and obtaining regular vitamin D serum levels.6
Recent investigations report incidence of fractures in SMA types 1-3 ranges from 20%4 to 38%3 patients. Fractures occur most often in the femur of SMA patients.3,4 Falls are a common precipitant of fractures, but fractures can occur after mild trauma or even spontaneously in patients with SMA.4 Treatment recommendations vary depending on the patient’s mobility. If a patient is not ambulatory, closed cast immobilization is sufficient for most fractures, but prolonged casting should be avoided because it can promote further muscle atrophy and osteoporosis.6 Experts advise that ambulatory patients with lower extremity fractures benefit from surgical stabilization as do non-ambulatory patients with hip fractures.6
References
1. Moore GE, Lindenmayer AW, McConchie GA, et al. Describing nutrition in spinal muscular atrophy: A systematic review. Neuromuscul Disord. Jul 2016;26(7):395-404.
2. Poruk KE, Davis RH, Smart AL, et al. Observational study of caloric and nutrient intake, bone density, and body composition in infants and children with spinal muscular atrophy type I. Neuromuscul Disord. Nov 2012;22(11):966-973.
3. Wasserman HM, Hornung LN, Stenger PJ, et al. Low bone mineral density and fractures are highly prevalent in pediatric patients with spinal muscular atrophy regardless of disease severity. Neuromuscul Disord. Apr 2017;27(4):331-337.
4. Vai S, Bianchi ML, Moroni I, et al. Bone and Spinal Muscular Atrophy. Bone. Oct 2015;79:116-120.
5. Martinez EE, Quinn N, Arouchon K, et al. Comprehensive nutritional and metabolic assessment in patients with spinal muscular atrophy: Opportunity for an individualized approach. Neuromuscul Disord. Jun 2018;28(6):512-519.
6. Mercuri E, Finkel RS, Muntoni F, et al. Diagnosis and management of spinal muscular atrophy: Part 1: Recommendations for diagnosis, rehabilitation, orthopedic and nutritional care. Neuromuscul Disord. Feb 2018;28(2):103-115.